The present invention relates generally to superconducting contacts to superconducting devices such as Josephson junctions, and specifically to the inclusion of a normal metal in a superconducting sandwich while maintaining superconducting contact. The invention is useful for superconducting connection of printed circuit lands having resistance to Josephson junction devices. The invention is useful for creating contact pads upon a substrate to which pads a totally superconducting, non-resistive, electrical connection, as by soldering, may be reliably made.
It is well known that normal metals, especially including the noble metal gold, may be deposited upon monolithic integrated circuitry operative at normal, non-superconducting, temperatures for the purposes of making electrical contact thereto. However, knowledge that all normal metals, including all noble metals, are, by definition, non-superconducting (i.e., "normal") precluded early attempts to utilize such normal metals, including the noble metals, as connective pads in the establishment of connections to monolithic superconducting, circuitry. Connection to superconducting circuitry upon a substrate was early made by compressively forcing a superconducting probe into contact with the superconductor of a substrate, and also by soldering using superconducting solder directly to such superconducting material upon a substrate. A problem with both the compressive connection and solder connection methods is that superconductors, such as niobium, are prone to oxidation which adversely affects the integrity of the compressive or soldered electrical interconnection thereto.
One measurement of electrical connection to superconducting circuit is shown in the article INDUCTANCE MEASUREMENTS OF SUPERCONDUCTING CHIP-TO-PACKAGE CONNECTORS SUITABLE FOR JOSEPHSON LSI TECHNOLOGY by D. C. Jones, D. J. Herrell and Y. L. Yao occurring in TEEE Transactions on Magnetics, VOL. MAG-15, NO. 1, Januray 1979. A connector is shown that bonds through an appropriate interface metallurgy (not specified, probably lead and impliedly not a noble metal because of the circuit design) a niobium pad on a superconducting chip to a similar pad on a superconducting module, the goal being the connection of such chip to such module. The bonded connection between the niobium pad upon the chip and that similar niobium pad upon the module is made by superconducting solder, forming thereby a superconducting connection. However, the connection from the niobium pads upon both the chip and the module to the etched regions (moat) on the chip--which etched regions realize the Josephson junction--is only via transmission lines of normal metal. This is because the only goal is low resistance interconnection of Josephson junctions upon a chip to superconducting transmission lines upon a module (and thence probably to another Josephson junction upon another chip), and not the creation of a totally superconductive interconnection.
The prior art emplacement of noble metal contact pads, specifically normal metal gold contact pads, upon a superconducting substrate, specifically a niobium substrate, by a combination of RF sputtering and photolithogrophy is abstracted in the report RF SPUTTERING OF GOLD CONTACTS ON NIOBIUM appearing in NASA Tech Briefs, Fall 1982, at page 104. Although the superiority of noble metal, mainly gold, contact pads in the making of reliable and stable electrical contacts to superconducting circuitry is taught, such teaching is only of the establishment of resistive, and not superconducting, contacts. Indeed, the typical utilization of normal, non-superconducting, metal even within monolithic superconducting circuitry is in the formulation of resistive interconnection between superconducting junctions or regions. Although resistive connections, including through connective pads of noble metal, can be made to superconducting circuitry through normal metal, in the prior art there exists only a hint that the presence of normal metal within a superconducting connection (which is not rendered non-superconducting thereby the presence of such normal metal) can be countenanced.
This hint arises from the construction of the Josephson junction itself wherein superconduction is obtained, via quantum mechanical tunneling, through a very, very thin layer, approximately 20 to 70 ANGSTROMS thickness, of a non-superconductor, such as silicon. It might be analogized from such a thin layer of silicon that possibly superconduction could be sustained through a very thin layer of normal metal, including the noble metals. Of course, certain questions present themselves regarding the sustaining of superconductivity through very thin normal metal layers. What geometries and/or processes might advantage themselves through such an effect? How thin must the normal metal layer be? At the required thinness of a normal metal layer is such layer thick enough to be useful, such as for a contact pad to, and through, which a totally superconducting connection may be made? What normal, and normal noble, metals are preferred?